This invention relates to the measurement of stresses or loads in fabrics of other flexible sheet materials under general states of biaxial stress. In particular, a transducer assembly with output from electrical resistance strain gages is developed to measure the biaxial state of stress in materials such as coated and uncoated fabrics and flexible sheet materials.
Heretofore, conventional methods of measuring stress or strain in fabrics have proved ineffective and/or cumbersome. Electrical resistance strain gages cannot be cemented directly to the uneven surface of the yarns, and in coated fabrics or other flexible sheet materials, such gages have a stiffening effect which causes disruption of the strain field being measured. For example, in coated fabrics, a gage causes the coating, to which it is cemented, to deform less than the underlying fabric, and hence indicate an incorrect strain. Mechanical extensometers do not provide an easily processed electrical output signal as would an electrical extensometer (for example using LVDT's), but both suffer from the need for a large length over which the strain is averaged and difficulties in installing more than one extensometer at a point to measure biaxial strains. Such extensometers also tend to be cumbersome and interfere with the stresses and strains present in the sheet. Other arrangements have used helically coiled strain gage wire to reduce the stiffening effect of the transducer, but such an arrangement is adversely affected by very slight bending or twisting, and measurement in more than one direction at a point is made difficult. Optical methods of measuring the distortion of a target imprinted on the sheet are not sufficiently sensitive for accurate determination of small strains, and they are cumbersome to use because they are not easily automated. All of the methods using extensometers or other transducers to measure strains in the flexible sheet require a knowledge of the material properties in order to compute the stresses. Often the biaxial stress-strain properties are not known well enough, or are not constant from one sample to another, to permit accurate computation of the stresses from the measured strains. In yet another arrangement, a transducer for load measurement uses a strain gage on a flat link which is secured at each end to a yarn. This arrangement can only be used to measure the load in one direction at a point. Typical disclosures of the prior art arrangements may be found in U.S. Pat. Nos. 3,118,301 and 4,038,867.